Tank Panzer Haubitze 2000

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In the early 20th century, the introduction of howitzers that were significantly larger than the heavy siege howitzers of the day made necessary the creation of a fourth category, that of "super-heavy siege howitzers". Weapons of this category include the famous Big Bertha of the German Army and the 15-inch (381 mm) howitzer of the Royal Marine Artillery. These large howitzers were transported mechanically rather than by teams of horses. They were transported as several loads and had to be assembled at their firing position.

These field howitzers introduced at the end of the 19th century could fire shells with high trajectories giving a steep angle of descent and, as a result, could strike targets that were protected by intervening obstacles. They could also fire shells that were about twice as large as shells fired by guns of the same size. Thus, while a 75 mm field gun that weighed one ton or so was limited to shells that weighed less than 8 kilograms, a 105 mm howitzer of the same weight could fire 15 kilogram shells. This is a matter of fundamental mechanics affecting the stability and hence the weight of the carriage. However, howitzers had a shorter maximum range than the equivalent gun.

US M198 gun-howitzer

As heavy field howitzers and light siege howitzers of the late 19th and early 20th centuries used ammunition of the same size and types, there was a marked tendency for the two types to merge. At first, this was largely a matter of the same basic weapon being employed on two different mountings. Later, as on-carriage recoil-absorbing systems eliminated many of the advantages that siege platforms had enjoyed over field carriages, the same combination of barrel assembly, recoil mechanism and carriage was used in both roles.

By the early 20th century, the differences between guns and howitzers were relative not absolute and generally recognized as follows:

• Guns – higher velocity and longer range, single charge propellant, maximum elevation generally less than 35 degrees.
• Howitzers – lower velocity and shorter range, multi-charge propellant, maximum elevation typically more than 45 degrees.

The onset of trench warfare after the first few months of the First World War greatly increased the demand for howitzers that gave a steep angle of descent, which were better suited than guns to the task of striking targets in a vertical plane (such as trenches), with large amounts of explosive and considerably less barrel wear. The German army was well equipped with howitzers, having far more at the beginning of the war than France.

Many howitzers introduced in the course of World War I had longer barrels than pre-war howitzers. The standard German light field howitzer at the start of the war (the 10.5 cm leichte Feldhaubitze 98/09) had a barrel that was 16 calibers long, but the light field howitzer adopted by the German Army in 1916 (105 mm leichte Feldhaubitze 16, see on the left) had a barrel that was 22 calibers long. At the same time, new models of field gun introduced during that conflict, such as the 77 mm field gun adopted by the German Army in 1916 (7,7 cm Feldkanone 16) were often provided with carriages that allowed firing at comparatively high angles, and adjustable propellant cartridges. In other words, there was a marked tendency for howitzers to become more "gun-like", while guns were taking on some of the attributes of howitzers.